270
FXUS66 KMTR 081147
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
347 AM PST Mon Dec 8 2025

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 117 AM PST Mon Dec 8 2025

 - Another round of fog this morning in the valleys across the Bay
   Area.

 - Fairly consistent weather pattern continues this week.

 - Next chance of rain around the 15th.

&&

.SHORT TERM...
Issued at 117 AM PST Mon Dec 8 2025
(Today and tonight)

Tule Fog remains in the Central Valley and there is slightly more
low stratus coverage across the Bay Area than this time last
night. There is a good chance for these clouds to expand down to
the surface as the the wind continues to calm through the morning.
The most likely locations to develop fog are the North Bay
Valleys, Interior East Bay Valleys, and the Santa Clara Valley.
There`s more uncertainty along the Bay side of San Mateo County.
Typically this area is less prone to fog than the cooler valleys,
but is the best set-up for it. Light NNE winds over the Bay don`t
suffer from the downslope drying of winds with a westerly
component. For example, historically the visibility at SFO drops
below 3 miles 8% of the time in December between 5AM and 10AM. If
we add a NNE wind direction constraint to these variables, the
probability increases to 17%. On the other hand, NW winds bring
this low visibility only 4% of the time. The otherwise clear
skies and long December nights also help with the radiative
cooling to generate this fog, and cloud top radiation helps to
sustain it. The complicating factor is the relatively warm water
in the Bay (55F) compared to the air temp (45F). This induces some
instability that will work to lift fog into a low cloud deck if
the low level winds don`t calm this morning. In the end, I`ll give
a 30% chance for fog in Eastern San Mateo, 50% for the Santa
Clara Valley, and 80% across the North and East Bay Valleys.
Otherwise the weather will be quite similar to the last couple
days as the fog and low clouds gradually clear through the day and
temperatures under the morning stratus struggle to get into the
60s in the afternoon.

&&

.LONG TERM...
Issued at 117 AM PST Mon Dec 8 2025
(Tuesday through Sunday)

The long term forecast is mostly boring. There is a hint of rain at
the end of the 7-day window, but let`s spend a minute discussing
this dry spell first. If you`ve read any of our AFDs in the last
week, you know it`s caused by a "ridiculously resilient ridge" over
California that is keeping the storm track well to our North. It`s
now been 17 days since San Francisco recorded any rain, and that
streak will very likely extend into the mid 20s. How unusual is
that? Well it turns out a long dry spell in the middle of the
rainy season is actually pretty common. There have been 54
instances of a dry spell lasting 24 days or more between November
and March since 1849. In other words, there is a 31% (54/176)
chance of getting a dry spell at least this long any given Winter.
The longest such streak was 60 days from November 17, 1876 to
January 15, 1877. We`re not expected to get anywhere near that
record as it looks like a pattern change is finally on the
horizon. More and more ensemble members are agreeing that the
ridge will finally break down with more a more active zonal
pattern bringing the jet stream and chances for rain back to the
Bay Area. 40/50 ECMWF ensemble members bring some rain to the Bay
Area next week, with an outside chance for heavier rain the
following weekend. We`ll wait and see for now, but our official
forecast finally has a decent probability of precipitation
starting on the 15th.

&&

.AVIATION...
(12Z TAFS)
Issued at 345 AM PST Mon Dec 8 2025

Tule fog pattern still strongly dominating the region. Based on
persistence, low confidence in actually clearing for North Bay
terminals, perhaps a few hours during the afternoon. High
confidence in clearing for Bay Area terminals (sans KLVK, lower
confidence), but stratus and fog will be patchy through sunrise.

Vicinity of SFO...Patch of stratus and fog moving along the
peninsula likely to persist through sunrise, clearing to VFR in the
few hours following. Otherwise quiet and clear through the afternoon
with stratus returning this evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...High confidence in VFR through the TAF
period. Light drainage offshore winds overnight with light onshore
flow during the afternoons.

&&

.MARINE...
(Today through Saturday)
Issued at 345 AM PST Mon Dec 8 2025

Moderate to fresh north to northwest winds continue for the
southern waters. Moderate seas with wave heights 6 to 8 feet
across the outer waters will subside to around 4 to 6 feet today.
Winds increase to a strong breeze Tuesday into mid week across the
outer waters. A new, long period northwesterly swell is  expected
by Wednesday, lasting through the latter half of the week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 AM PST Tuesday for Pt Pinos to Pt
     Piedras Blancas 0-10 nm.

     Small Craft Advisory from 3 PM this afternoon to 3 AM PST
     Wednesday for Pigeon Pt to Pt Pinos 10-60 NM-Pt Arena to Pt
     Reyes 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Behringer
MARINE...Behringer

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